1. Field of the Invention
This invention relates to a process for preparing lubricating oils. More particularly it relates to an improved process for dewaxing refrigerator oils.
2. Description of the Prior Art
The invention relates to the urea-dewaxing of mineral oils containing very small amounts of n-paraffins. More particularly, the invention relates to the production of refrigerator oils from such low paraffin content mineral oils.
It is known to dewax hydrocarbon mixtures or mineral oil distillates by reacting them with urea to form solid adducts of the n-paraffins and separating these adducts from the dewaxed mineral oil. This process is called urea dewaxing and has been used in different embodiments for the large-scale refining of mineral oil. Usually in this process, the mineral oil distillate charge is diluted with urea dewaxing solvent, i.e., an organic solvent or mixture of solvents in order to reduce the viscosity of the reaction mixture, to improve its pumping and mixing properties, and to increase the degree of separation of the adduct and the dewaxing selectivity. In most cases, the same solvent, or solvent mixture, is used for washing the separated adducts and, if desired, for extracting the n-paraffins from the adduct. The solvent or solvent mixture, should not form adducts with urea under the reaction conditions being employed. Conventional solvents include lower aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons such as, dichloromethane, and the like.
Usually, the urea is dissolved in water or methanol, the solution having been saturated at a temperature above the predetermined reaction or adduct-forming temperature. The amount of urea required for adduct formation is from 3.5 to 4 parts by weight per part by weight of n-paraffin to be separated from the mineral oil. It has also been proposed to use urea in crystalline form, preferably finely divided. In this case the formation of adduct progresses at a very slow rate, therefore usually minor amounts of activators such as water, methanol, ketones, and other urea-dissolving agent are added.
It is also known to initiate or accelerate the adduct-forming process by introducing a seed adduct. Another known method of enhancing the formation of adduct is by vigorously mixing the reaction mixture.
Refrigeration oils are used in the refrigeration and air conditioning industry to provide lubrication for refrigeration compressors. Two important characteristics of refrigeration oils are chemical and thermal stability. Chemical stability applies to chemical inertness to other system components at elevated temperatures as well as resistance to oxidation. These components include ammonia, sulfur dioxide, chlorinated hydrocarbons and the Freons, e.g., Freon 12 (dichloro-difluoro methane). Thermal stability refers to the resistance of the oil to degradation under thermal stress and thereby form deposits which coat heat exchanger surfaces or plug the expansion valve.
There is a need in the art for a processing sequence which affords a refrigeration oil with chemical and thermal stability. Production of such an oil is particularly difficult when the distillate oil charge stock comprises only minor amounts or just traces of n-paraffins.
It is the object of this invention to improve the urea dewaxing process so that very small amounts of n-paraffins contained in mineral oil distillate and similar hydrocarbon mixtures may be removed therefrom in a simple relatively rapid manner, and that the removal is practically complete and is preferably performed in a continuous operation. Another object is to produce a lubricating oil for refrigerating machines from a naphthenic mineral oil distillate.
In this regard, the present invention is an improvement in the art of urea dewaxing. Another contribution to the art of urea dewaxing is U.S. Pat. No. 3,945,912.